refactor: make cel_serial_read non-blocking

Drop timeout_ms; read now returns immediately with whatever
data is available (0 if none), so callers don't block the rest
of their loop waiting on serial I/O.

telemetry's poll loop now sleeps interval_ms itself between
empty reads via a small sleep_ms helper.

celrs/serial.c

@@ -37,12 +37,11 @@ void cel_serial_close(cel_serial_port* port) {
     free(port);
 }
 
-size_t cel_serial_read(cel_serial_port* port, uint8_t* buf, size_t len, int timeout_ms) {
+size_t cel_serial_read(cel_serial_port* port, uint8_t* buf, size_t len) {
     (void)port;
     (void)buf;
     (void)len;
-    (void)timeout_ms;
-    /* TODO: platform-specific read */
+    /* TODO: platform-specific non-blocking read */
     return 0;
 }
 

celrs/serial.h

@@ -15,8 +15,9 @@ cel_serial_port* cel_serial_open(char const* path, int baud_rate);
 /* Close and free the serial port */
 void cel_serial_close(cel_serial_port* port);
 
-/* Read up to len bytes. Returns bytes read, 0 on timeout/error. */
-size_t cel_serial_read(cel_serial_port* port, uint8_t* buf, size_t len, int timeout_ms);
+/* Read up to len bytes without blocking.
+ * Returns bytes read immediately available, or 0 if none/error. */
+size_t cel_serial_read(cel_serial_port* port, uint8_t* buf, size_t len);
 
 /* Write data. Returns bytes written, 0 on error. */
 size_t cel_serial_write(cel_serial_port* port, uint8_t const* buf, size_t len);